As the U.S. marine industry pushes toward decarbonization, our domestic fleet will be the first to adopt and experiment with different propulsion methods. For the majority of passenger vessel operations, the most viable and fully developed option for substantively reducing carbon emissions will be all-electric propulsion—where it can be practically implemented.
However, certain vessels may not be able to accommodate marine battery systems due to weight or space constraints. For others that operate on longer routes, have highly variable operating profiles or serve rural areas where the addition of shoreside charging infrastructure is impractical, all-electric propulsion may not be achievable. These vessels will need to consider alternative liquid or gas fuels.
Of course, there is no panacea alternative fuel for the marine industry, and at present, none of the alternative fuels currently available are entirely sustainably produced. However, plans to shift production towards net-zero impact are beginning to take shape. Operators weighing alternative-fuel options for passenger vessels should consider utilizing dual-fuel or “multi-fuel” internal combustion (IC) engines in combination with fuels that are on the path to green production.
While fuel-cell technology appears very promising for decarbonization of marine vessels, the regulatory framework for incorporating this technology in vessel design is still evolving. Fuel cells also introduce a level of cost and technical risk that make them a non-starter for many operators—at least for now. For operators looking for a manageable near-term transition to alternative fuel that is minimally disruptive to their current business model, methanol is a good choice.
A Viable Replacement
For many vessels, methanol is a viable diesel replacement. Like diesel, methanol is a liquid fuel that can be carried in hull-integrated tanks and used in modern compression -ignition engines. It is compatible with our existing marine-fuel infrastructure, and there is no need for major vessel modifications to accommodate its use.
Perhaps more importantly, with current pricing trends, methanol is approximately the same cost per BTU as diesel. The combustion of pure methanol produces no sulfur oxides (SOx) and significantly lower nitrogen oxides (NOx) and particulate-matter emissions when compared to diesel. Methanol is also water soluble and biodegradable.
One of the primary reasons why methanol will be a viable diesel replacement for many operators is because it remains in a liquid state in ambient conditions. It is not a cryogenic fuel or a pressurized gas, so bunkering and storing it onboard is relatively simple, and means no significant change in operations for vessel owners.
This ease of handling makes methanol a particularly attractive contender for broad adoption by the marine industry. Its production in the U.S. and around the world is already long established, and the infrastructure necessary to move and store methanol is largely in place.
Moreover, methanol can be produced from a wide range of feedstocks, which means that operators can achieve carbon-neutral operations, provided the methanol is sustainably sourced. Lastly, a regulatory framework around methanol-powered vessels is starting to emerge and the requirements are decidedly less burdensome than for more volatile marine fuels such as liquified natural gas and ammonia.
A methanol-ready passenger vessel can be built today. Several major engine manufacturers already offer methanol-compatible internal combustion engines, while others have plans to commercially offer such engines in the near future.
This means that operators interested in retrofitting, as opposed to building new, can often make the switch to methanol without having to make major modifications to their existing fleet.
But methanol is not without its challenges. It is highly corrosive, which means adopters will have to be careful about the materials and coatings they select for storage tanks. Because the energy density of methanol is roughly one half that of diesel (methanol has 45% of the energy by volume and 47% of the energy by mass), storage tanks will need approximately twice the volume to achieve the equivalent caloric value.
Because methanol is a Class-A liquid with a lower flashpoint than diesel, methanol storage tanks will require inert-gas blanketing. Hazardous area zone classifications will be required around fuel preparation spaces, fuel tank vents and enclosed spaces which contain fuel pipes. Some fuel piping will be required to be of double-wall construction or contain an airtight duct around it.
Depending on the vessel arrangements and the size of the hazardous zones, complying with these requirements could be challenging for existing vessels looking to convert.
Methanol is also less viscous than diesel-oil and does not afford the same properties of lubricity. For some methanol combustion engines, fuel additives will be required for lubricity and to enhance ignition.
Most of today’s methanol is produced by steam-reforming natural gas, known as gray methanol, or gasifying coal, known as brown methanol, and does not offer much—if any—of a reduced carbon footprint over burning diesel. Carbon capture and cleaner sources of hydrogen can be incorporated into production and the resulting “blue methanol” has a lower carbon footprint than gray or brown methanol.
Green methanol includes both biomethanol, or methanol produced from biomass and other non-fossil sources, and e-methanol, methanol produced from green hydrogen and captured carbon dioxide from industrial sources. It can be very close to net carbon neutral, and numerous projects to produce this fuel are in the works around the globe.
While green methanol is not yet produced in sufficient quantities to support widespread adoption by industry, it may be in the future.
In summary, the challenge operators face switching from fossil fuels to propulsion systems with zero or carbon-neutral emissions is significant, and though battery power offers one obvious solution, it is simply not feasible for all vessel types and operating profiles.
While the adoption of fuel cells may not appeal to many operators at this stage of development, methanol (CH3OH) is a hydrogen carrier. It is regarded by many as a logical transition fuel as reformer and fuel cell technologies continue to mature. For vessels that can’t go all-electric, methanol is the best emergent option for the elusive “future-proof” design, providing a pathway to carbon neutrality that is adaptable for hydrogen propulsion when operators are ready.
The reality of decarbonization is that an industry sea change will not happen overnight. IC engines are not going away anytime soon and, at least in the near term, there is a need for a liquid fuel alternative to marine diesel. In the current environment, the best path forward is to adopt a propulsion system that can support a transition from diesel to an alternative fuel at a pace that aligns with the maturation of technology and the establishment of a regulatory framework that is predictably and consistently applied.
Shipping giant Maersk’s plan to build eight methanol-diesel-powered container vessels is testament to the fact that the momentum behind methanol as a transition fuel is growing. With its availability, compatibility with existing infrastructure and potential for sustainable production, methanol offers operators an opportunity for a strategic compromise.
Morgan Fanberg is president of Glosten, a full-service naval architecture and marine consulting firm. A graduate of the US Merchant Marine Academy, he is a professionally licensed engineer offering more than 20 years of marine-industry experience specializing in marine mechanical and electrical systems. He will be a featured speaker at the 2022 Ferries Conference, being held at the Seattle Renaissance Hotel on Sept. 22, 2022. Photo via Glosten. www.FerriesConference.com